NUCHAL TRANSLUCENCY SCAN AT 11–13 WEEKS
Nuchal translucency, combined with the PAPP-A test, was until non-invasive prenatal testing appeared the most advanced prenatal population screening test for the commonest chromosomal abnormalities, such as Down syndrome and trisomies 18 and 13. It is performed between the 11th and the 14th week of pregnancy.
The purpose of the scan is:
To date the pregnancy accurately. This is particularly important for women who do not remember their last period, who have irregular cycles, or who conceived while breastfeeding or shortly after stopping contraception — and especially if they have not had an early pregnancy scan. By measuring the crown–rump length (CRL) of the fetus we can date the pregnancy accurately and establish the expected date of delivery.
To establish the number of fetuses and, in a multiple pregnancy, the chorionicity. This is the only stage at which chorionicity can be determined reliably; later on there are cases where it cannot be established accurately.
To diagnose certain major congenital abnormalities. The 11–13+6 week scan (nuchal translucency) also includes an examination of the baby's anatomy. We look at the skull, the brain, the lenses of the eyes, the palate, the upper and lower limbs, the heart, the stomach, the kidneys, the bladder, the spine, the anterior abdominal wall and the umbilical cord. The first-trimester anatomy scan is performed using specific ultrasound views, as laid down in the protocols of the Fetal Medicine Foundation (FMF), which have been shown to have the highest detection rate. Significant abnormalities may be visible at this gestational age, but the 20-week scan is also essential.
To calculate the risk of preeclampsia and of intrauterine growth restriction. In every patient, at the end of the examination, the blood flow in the mother's uterine arteries is measured by Doppler ultrasound.
The result of this test can tell us which women are at increased risk of developing preeclampsia, mainly before 34 weeks, and which fetuses are at risk of growth restriction, mainly in the third trimester.
To calculate the risk of preterm birth. In every patient who wishes it, the length of the cervix is measured transvaginally at the end of the examination. We know that during pregnancy the cervix should be long and closed. In women whose cervix is found to be short, the risk of preterm birth is increased. Management in that case depends on the problem.
To assess the risk of Down syndrome and other chromosomal abnormalities. Every woman should receive an estimate of her own individual risk for this pregnancy.
The test that calculates, with the greatest accuracy, the individual probability or risk that the fetus is affected by the three commonest chromosomal abnormalities — Down, Patau and Edwards syndromes (trisomies 21, 13 and 18 respectively) — is carried out at the 11–13+6 week scan.
The risk calculation takes into account:
- the amount of fluid at the back of the fetus's neck (the nuchal translucency)
- the presence of the nasal bone
- blood leakage at the tricuspid valve of the fetal heart
- the blood flow in the fetal ductus venosus
- the fetal heart rate
- whether or not there is any anatomical abnormality of the fetus
It should be stressed that, to achieve these detection rates, the ultrasound examination of the fetus must follow the protocols of the Fetal Medicine Foundation and be performed by doctors licensed by the Foundation, who are audited by it annually in order to have their licence renewed. The hormone levels in the mother's blood must likewise be measured by laboratories that hold the relevant certification and have the equipment specified by the Fetal Medicine Foundation.
The Embryocosmos centre is able to calculate the combined risk using the PAPP-A test. The blood test is taken shortly before the scan, so that when the scan is complete (after 120 minutes) you receive the final result of the examination, including the PAPP-A.
After the scan, on the basis of all the factors above, the estimated risk of Down syndrome will be discussed with you. Only you can decide whether you wish to have an invasive diagnostic test.
the mother's age; the history of previous pregnancies; and two biochemical markers produced during pregnancy by the early placenta: beta human chorionic gonadotropin (β-hCG) and pregnancy-associated plasma protein A (PAPP-A). Their levels are measured from a blood sample taken from the mother and give us a hormonal and biochemical profile of the pregnancy. The nuchal translucency scan becomes more sensitive and more accurate when it is combined with the PAPP-A test: in this way its accuracy reaches 90%, with only 3% false positive results. Nuchal translucency on its own detects about 75% of fetuses with Down syndrome. That figure rises to 90% when, alongside the nuchal translucency, we examine some of the other ultrasound markers (nasal bone, tricuspid valve, ductus venosus), and to 95–97% when the biochemical tests (PAPP-A and free β-hCG) are included as well.
As regards chromosomal abnormalities, it must be stressed that the only way to know with certainty whether or not the fetus has a chromosomal abnormality is to perform an invasive procedure such as chorionic villus sampling (CVS) or amniocentesis, in which the genetic material of the fetus is examined. These invasive procedures, however, carry a risk of miscarriage of about 1%. For that reason the decision whether or not to proceed with such a procedure is best based on a test that calculates the individual probability of a chromosomal abnormality in the fetus.
It is worth noting that even when the fetus shows one of these ultrasound markers and has an increased probability of a chromosomal abnormality, this does not necessarily mean that there is anything wrong: the baby may turn out to be completely normal.
Whether or not you have decided to have an invasive test, a detailed scan at 20 weeks is recommended to check for structural abnormalities in the fetus.
Where the fetus is in a suitable position, images are also taken using three-dimensional (3D) ultrasound.